1. Field of the Invention
The present invention relates to an imaging element, an imaging device, an endoscope, an endoscope system, and a method of driving the imaging element.
2. Description of the Related Art
In general, each unit pixel in an imaging device having a CMOS (Complementary Metal-Oxide Semiconductor) imaging element includes at least one photoelectric converter (photodiode), a charge converter, a transfer transistor, a charge converter reset transistor, a pixel source follower transistor, and a pixel output switch transistor.
A charge accumulated in the photoelectric converter is read by the transfer transistor, subjected to voltage conversion by the charge converter, and then output to a vertical transfer line through the pixel source follower transistor and the pixel output switch transistor. Such imaging device performs pixel selection by controlling drive of the pixel output switch transistor.
Various contrivances have been tried in recent years to achieve downsizing of the imaging element. As one of the contrivances, it has been proposed to simplify a pixel configuration by omitting the pixel output switch transistor that configures each unit pixel (e.g., refer to Japanese Patent Application No. 2004-172950).
When a synchronization signal is received from outside to perform pixel selection in an imaging element from which the pixel output switch transistor is omitted, it has been adapted to perform the pixel selection operation upon reception of the synchronization signal from outside and perform a de-selection operation on the selected pixel after reading the selected pixel. It is thus possible that, when the synchronization signal is input at a timing different from a timing at which the synchronization signal is to be input originally, a pixel selected when the synchronization signal is input remains selected and selected while overlapping with a pixel that is newly selected upon reception of the synchronization signal, because the de-selection operation of the pixel being read is not performed properly. As a result, an imaging signal corresponding to at least one frame overlaps, causing a possibility of a frame drop occurring across a plurality of frames.
When such imaging element is used in an endoscope system, the frame drop occurring in such situation may possibly be an obstacle to an observation of a subject.
Moreover, the endoscope system is often used in combination with a device such as an electric knife generating noise, which is in some cases misrecognized as a synchronization signal from outside by the imaging element. When the imaging element is provided at a distal end of an insertion unit of the endoscope and receives the synchronization signal through the insertion unit, for example, the signal is easily affected by the noise generated by the electric knife or the like through the long insertion unit, whereby misrecognition of the synchronization signal due to the disturbance may possibly cause the aforementioned frame drop as well.
There is a need for an imaging element, an imaging device, an endoscope, an endoscope system and a method of driving the imaging element that can prevent a frame drop caused by a synchronization signal input at a timing different from a timing at which the synchronization signal is to be input originally.